Semi-hydrotropic chemical lignocellulose pulping process



United States Patent C SEMI-HYDROTROPIC CHEMICAL LIGNOCELLU- LOSEPULPING PROCESS Application January 7, 1955, Serial No. 480,400

14 Claims. (Cl. 92-9) This invention relates to a process for makingcellulosic pulps and the product therefrom. More particularly, itrelates to a process for cooking lignin containing cellu losic material,without appreciable loss in weight of the lignin content of the materialdue to solvent action, to render said material suitable for rapiddisintegration into its fibrous components.

Most chemical processes for treating ligno-cellulosic material to obtainthe cellulose fibers therefrom, require a cooking with a chemicalsolution at an elevated temperature to induce a solvent action upon thelignin. The dissolving of the lignin frees the cellulose fibers butsince lignin constitutes about 20 to 30 percent of the ligno-.cellulosic material, it is apparent that a considerable portion of thewood is not available in this type of chemical pulp.

It is an objective of this invention to provide a process for pulpinglignocellulose material which will yield a high percent of pulp, forexample 90 percent of the original material being pulp.

It is another objective of this invention to provide a process requiringlittle equipment cost and which may be carried out in open tanks. v

It is a further objective of this invention to provide a continuousprocess for pulping cellulosic rawmaterials, It is another objective toprovide a chemical pulp having substantiallyall the original lignin andhaving a high opacity.

It is another objective .to provide a chemical paper pulp having a highabsorbency value and which is resistant to becoming brittle due to lightand which does not 'discolo'r appreciably due to ultra-violet light.

It is another objective of this invention to provide a paper pulp makingprocess wherein the cooking solution maybe used a plurality of times andwith successively increasing efiectiveness.

It is yet another objective of this invention to provide a chemicalpaper pulp having substantially natural lignin and in an amountsubstantially that present in the raw lignocellulose material to obtainsaid pulp.

These and other objectives of this invention will betion taken inconjunction with a flow sheetdiagram showing the continuous pulping oflignocellulose material by the inventive process.

According to thisinvention, a chemical paper pulp is produced whichcontains substantially all the lignin origi-- originally present in thelignocellulose material are dissolved by this inventive process. As aresult, a high yield of chemical pulp, for example 90 percent of theweight of the original lignocellulose is obtainable by the process ofthis invention. The commonly used sulfite or sulfate processes gives apulp yield of only about 50 percent of the original weight of thelignocellulose material.

Substantially any lignocellulose material may be pulped by this process,for example, the various woody plants, such as poplar, maple, etc. thevarious grasses such as kunai and wheat'straw, also bamboo, kenaf andthe like type of lignocellulose may be pulped by this process, as wellas the bagasse obtained from the sugar cane industry. The pulp obtainedby the process of this invention is a chemical pulp. It contains about/2 the pentosan content of the original lignocellulose just as is thecase for chemi cal pulps. Yet it possesses many of the desirablequalities of groundwood, which contains substantially 100 percent of'the pentosans, without possessing the undesirable groundwood propertiessuch as turning yellow or brow on exposure to ultra-violet light.

The pulp obtained in this invention is a novel product and inventive assuch, since it possesses a fibrous texture and strength characteristicssimilar to that of the commonly available chemical sulfite and sulfatepulps, yet it also possesses the high opacity of groundwood pulps.

Chemical pulps for example, bleached sulfate pulp, ex-

- trary, a sheet or paper made from chemical pulp produced by thisprocess compares very well to the tear test and burst test properties ofpaper made from sulfite or sulfate pulp.

In short, the high lignin pulp of this invention is a' chemical pulpexhibiting qualities comparable to substantially'ligniu-fr ee sulfite orfulfate chemical pulp. Thus, while the percentage of chemicalconstituents of this pulp approximates groundwood pulp, the desirablephysical properties of this new pulp approximates that of the commonlyavailable unbleached chemical sulfite or sulfate pulps. The chemicalpulp of this invention is distinguished in that it contains thenaturally occurring lignin in substantially its natural chemicalcondition and in substantially the percentage amount occurring in thelignocellulose raw material used.

- In U. S. Patent No. 2,308,564 a process employing a concentrated (25to 40 percent) solution of hydrotropic salts was employed to dissolveout substantially all of the lignin present in the lignocellulose rawmaterial being treated.

come apparent upon reading the following descriptive dis: closure ofseveral illustrative embodiments of this inven- The chemical pulpprocess of this present invention is different in kind sincesubstantially no lignin is dissolved. In this present process a weakhydrotropic solution is used. Such a weak solution contains only about 1to about 5 percent of a hydrotropic salt solution, and by the termhydrotropic salts as used herein, the term is intended to include thehydrotropic acids which may be pentosan content originally present,whereas the prior art hydrotropic process is based upon the removal oflignin.

In other words, in the instant hydrotropic process the cellulose fibersare liberated by removing or weakening the cementing or bonding actiondue to pentosans whereas in-the prior art hydrotropic' process thefibers are Patented Feb. 26, 1957;

shown in the drawing, as for example, two refiners in series'may be usedin place of the single refiner 26. Also Where bamboo, kenaf, bagasse andsome woods are pulped the product from the refiner 26, being creamcolored, it is useable as such without bleaching.

This invention will be better understood by reference to the followingillustrative examples, which show the broad latitude of the scope ofthis invention.

Example I One part by weight of bagasse was heated with twelve parts byweight of a 1 percent aqueous solution of sodium Xylenesulfonate for twohours at 95 C. The pulp was removed from the cooking solution, thenwashed with water, refined and bleached using three treatments of dilutehypochlorite solution with intermediate washings with water. Theresulting product was refined and made into paper. This paper comparedwith paper made from 100 percent commercially produced spruce groundwoodpulp, a high opacity pulp. The paper made from 100 percent sprucegroundwood pulp-showed an opacity of only 92 percent whereas the papermade from 100 percent bagasse pulp made by the hydrotropic process ofthis invention showed an opacity figure 4 percent higher or 96 percent.Standard newsprint of 80 percent groundwood and 20 percent sprucesulphite shows an opacity of about 90 percent and whole bagasse bleachedkraft chemical pulp shows an opacity of about 79 percent.

Analysis of the bagasse pulp showed a lignin content of 17 percent and apentosan content of 21 percent. The original bagasse composition was 19percent lignin and 27 percent pentosans. Therefore, there was 2 percentremoval of lignin and 6 percent removal of pentosans. The used solventhad a pH of about 4 and was the color of very weak tea.

Example II Bamboo chips (Phyllostachy bambusoides) were heated with tentimes its weight of a 2 percent aqueous solution of sodiumxylenesulfonate which had been previously used for the same purpose andwhich had a pH of about 4. The heating was in an open kettle for threehours at 95 to 100 C. The residual chips were washed with water andrefined but not bleached, since the resulting bamboo pulp was light incolor and sufliciently opaque to be used in newsprint in place ofgroundwood. The residual light colored cooking liquor had a pH of 4 andwith a slight strengthening back to 2 percent with additionalhydrotropic salt it was ready for reuse. Analysis of the pulp showedthat the bamboo pulp had substantially the same lignin content as thebamboo chips but only about of the original pentosan content. The yieldof pulp was 87 percent of the bamboo chips taken.

Example III One part by weight of the woody residue left from the stalksof kenaf after removal of the fibrous bark was heated at 95 C. for threehours with ten parts by weight of a 1 percent aqueous sodiumxylenesulfonate solution to which suflicient Xylenesulfonic acid hadbeen added to bring the solution to a pH of 3. The resulting pulp, afterwashing and refining but with no bleaching, was almost white andpossessed a high opacity.

Example 1V One part of weight of chips of aspen poplar (Populustremuloides) were heated for 4 hours at 100 C. with 7 parts by weight ofa 2 percent sodium cymenesulfonate solution made acid to a pH of 4 byaddition of a small amount of cymenesulfonic acid. After washing andrefining, the pulp was a light tan color which could easily be bleachedto a light cream color by use of a hypochlorite or a peroxide solution.The resulting pulp was of excellent quality. It was obtained in highyield and it had a lignin content of 22 percent and only 14 percentpentosans.

yellow birch (Betula lutea) were poured into a tank con taining tenparts by weight of a 2 percent solution of sodium xylenesulfonate of pH6 held at 98 C. A stainless steel weight was then placed upon the chipsto keep them submerged in the tank. In twenty minutes the pH ofthesolvent had changed to 4.5 and in an hour to 4.0.

Heating was continued for a total period of three hours at 97 to 100 C.The liquid was then removed, the chips'roughly washed with cold water,and then refined by passage at successively closer settings, three timesthrough a small Sutherland disk refiner. The resulting pulp Was screenedon a 12 cut screen and the fines rescreened on an 8 cut commercial typescreen. Only a small proportion of screenings were obtained. The pulpthrough the 8 cut screen was bleached at a pH of 9 with about 8 percentof chlorine in the form of hypochlorite solution. The resulting birchpulp on analysis showed a lignin content within 1 percent of thepercentage of lignin present in the untreated chips but the percentageof pentosans in the pulp had been reduced by some 8 percent compared tothe amount in the original birch chips. The yield pulp was 88 percentand the color of the pulp was a cream white. The pulp possessed highopacity. In general it was not as strong or as permanent a type of pulpas that made by the sulphite process but it was of better color andquality than would have been obtained from yellow birch by thegroundwood process.

Having read the disclosure hereinabove it is apparent that a genericinvention has been made in that the lignocellulose pulp process of thisinvention pulps lignocellulose by the selective removal of pentosanswithout any substantial removal of lignin, whereas the lignocellulosepulping processes prior to this invention eifected the pulping processby substantially the complete removal of lignin.

It is also apparent that those skilled in the art, having now beenintroduced to this invention, will readily perceive various possiblechanges, but all these changes being within the skill of the art aredeemed to be within the scope of this invention and covered by theclaims herein.

It is further apparent that this process produces a novel pulp, in largepart suggestive of the chemical pulps as to pentosan content, color andstrength characteristics, but also the novel pulp is suggestive ofgroundwood in that it is highly opaque and has good absorbency.

In the following claims by a dilute solution is meant one having aconcentration of about one to about five percent of the hydrotropic saltin water and by cooking is meant a process similar to that of preparingfood by boiling food in water in an open vessel or a pressure cooker.

I claim:

1. The process for producing a lignocellulose pulp from pentosancontaining lignocellulose material said pulp having substantially thelignin content of the lignocellulose material comprising cooking saidstarting material with a dilute 1 to 5 percent aqueous solution of ahydrotropic compound selected from the group consisting of benzoic acid,salicylic acid, xylenesulfonic acid, cymenesulfonic acid, phenolsulfonicacid, toluenesulfonic acid and their alkali metal salts until asubstantial amount of the pentosans is removed.

2. The process of claim 1 wherein the temperature of cooking is about100 C. and wherein from A to /h of the pentosans are removed.

3. The process of claim 1 wherein the temperature of cooking issubstantially the boiling point of the aqueous hydrotropic solution.

4. The process of claim 1 wherein the cooking is done in an open vesselat atmospheric pressure.

5. The process of claim 1 wherein the cooking is done 7 in a closedvessel at a temperature and pressure above that of normal atmosphericconditions.

6. The process of claim wherein the time of cooking is about one hour.

7. The processof claim 1 wherein the cooking is done under refluxcondenser conditions whereby the cooking vapors are condensed.

8. The process for producing a lignocellulose pulp from pentosancontaining lignocellulose material said pulp having substantially thelignin content of the lignocellulose material comprising cooking saidstarting material with a dilute l to 5 percent aqueous solution of ahydrotropic compound selected from the group consisting of benzoic acid,salicylic acid, xylenesulfonic acid, cymenesulfonic acid, phenolsulfonicacid, toluenesulfonic acid and their alkali metal salts at from 95 C. to100 C. until from /3 to /z of the original pentosan content is removed.

9. The process of claim 8 wherein the time of cooking is a plurality ofhours.

10. The process of claim 8 wherein the hydrotropic compound is sodiumxylenesulfonate.

11. The process of claim 10 wherein the lignocellulose material isbagasse.

12. The process of claim 10 wherein the lignocellulose material is wood.

13. The process of claim 8 wherein the cooking of the lignocellulosematerial is done by moving said material continuously through saidaqueous cooking solution until /3 to /2 of the pentosan content of thelignocellulose material is removed, and thereafter removing the so 9cooked'inate'rial continuously from said cooking solution. 14. Theprocess of claim 13 wherein the cooking is done in ariopen vessel andwherein dilute aqueous cooking solution is added continuously to saidvessel to make up for solution losses due to removal of cooked materialfrom said vessel.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Hydrotropic Solutions as Solvents of Lignin, by Pelipetz,Columbia Univ., New York, pp. 10, 14, 15, 18, 19, 24, 25, and (1937).Copy in Div. 67.

Aronovsky et al.: Industrial and Engineering Chemistry, November 1936,pp. 1270-1276.

Casey: Pulp. and Paper, published by Interscience Publishers, New York,1952, volume I, p. 205.

1. THE PROCESS FOR PRODUCING A LIGNOCELLULOSE PULP FROM PENTOSANCONTAINING LIGNOCELLULOSE MATERIAL SAID PULP HAVING SUBSTANTIALLY THELIGNIUM CONTENT OF THE LIGNOCELLULOSE MATERIAL COMPRISING COOKING SAIDSTARTING MATERIAL WITH A DILUTE 1 TO 5 PERCENT AQUEOUS SOLUTION OF AHYDROTROPIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF BENZOIC ACID,SALICYLIC ACID, XYLENESULFONIC ACID, CYMENESULFONIC ACID, PHENOLSULFONICACID, TOLUENESULFONIC ACID AND THEIR ALKALI METAL SALTS UNTIL ASUBSTANTIAL AMOUNT OF THE PENTOSANS IS REMOVED.